Edit the migrations:

# db/migrations/20171024083639_create_users.rb
Hanami::Model.migration do
  change do
    create_table :users do
      primary_key :id
      column :name, String, null: false
      column :created_at, DateTime, null: false
      column :updated_at, DateTime, null: false
    end
  end
end

# db/migrations/20171024083725_create_avatars.rb
Hanami::Model.migration do
  change do
    create_table :avatars do
      primary_key :id
      foreign_key :user_id, :users, null: false, on_delete: :cascade
      column :url, String, null: false
      column :created_at, DateTime, null: false
    end
  end

Now we can prepare the database:

# lib/bookshelf/repositories/user_repository.rb
class UserRepository < Hanami::Repository
  associations do
    has_one :avatar
  end
  def create_with_avatar(data)
    assoc(:avatar).create(data)
  end
  def find_with_avatar(id)
    aggregate(:avatar).where(id: id).map_to(User).one
  end
end

We have defined only for the operations that we need for our model domain. In this way, we avoid to bloat UserRepository with dozen of unneeded methods.

Let’s create an user with an avatar single database operation:

repository = UserRepository.new
user = repository.create_with_avatar(name: "Luca", avatar: { url: "https://avatars.test/luca.png" })
  # => #<User:0x00007fa166ac8550 @attributes={:id=>1, :name=>"Luca", :created_at=>2017-10-24 08:44:27 UTC, :updated_at=>2017-10-24 08:44:27 UTC, :avatar=>#<Avatar:0x00007fa166ac35c8 @attributes={:id=>1, :user_id=>1, :url=>"https://avatars.test/luca.png", :created_at=>2017-10-24 08:44:27 UTC, :updated_at=>2017-10-24 08:44:27 UTC}>}>
user.id
  # => 1
user.name
user.avatar

Because we haven’t explicitly loaded the associated record, user.avatar is nil. We can use the method that we have defined on before (#find_with_avatar):

user = repository.find_with_avatar(user.id)
  # => #<User:0x00007fa166a71048 @attributes={:id=>1, :name=>"Luca", :created_at=>2017-10-24 08:44:27 UTC, :updated_at=>2017-10-24 08:44:27 UTC, :avatar=>#<Avatar:0x00007fa166a70328 @attributes={:id=>1, :user_id=>1, :url=>"https://avatars.test/luca.png", :created_at=>2017-10-24 08:44:27 UTC, :updated_at=>2017-10-24 08:44:27 UTC}>}>
user.avatar
  # => #<Avatar:0x00007fa166a70328 @attributes={:id=>1, :user_id=>1, :url=>"https://avatars.test/luca.png", :created_at=>2017-10-24 08:44:27 UTC, :updated_at=>2017-10-24 08:44:27 UTC}>

This time user.avatar has the associated avatar.

# lib/bookshelf/repositories/user_repository.rb
class UserRepository < Hanami::Repository
  # ...
  def add_avatar(user, data)
    assoc(:avatar, user).add(data)
  end
  def remove_avatar(user)
    assoc(:avatar, user).delete
  end
  def update_avatar(user, data)
    assoc(:avatar, user).update(data)
  end
  def replace_avatar(user, data)
    assoc(:avatar, user).replace(data)
end